Texturing of magnetic disk substrates

ABSTRACT

A method for texturing a substrate for a magnetic disk comprises abrading the substrate using nano-sized diamond particles (e.g. having a diameter less than or equal to 20 nm). A magnetic layer is then deposited over the substrate. Even when the texture is extremely smooth (e.g. less than about 2.2 Å Ra), the disk still exhibits good Hc and MrT orientation ratios, SNR and PW50.

BACKGROUND OF THE INVENTION

This invention pertains to methods for texturing magnetic disksubstrates and the resulting substrates. This invention also pertains tomethods for manufacturing magnetic data storage media and the resultingmedia.

Magnetic disks (e.g. disk 1 of FIG. 1) are manufactured by a) polishinga substrate; b) texturing the substrate; and c) depositing a set oflayers on the substrate, e.g. by sputtering or other vacuum depositiontechnique. The substrate typically comprises an aluminum alloy 2 coatedwith an electroless-plated nickel phosphorus (“NiP”) alloy 3. The layerscan comprise one or more seed and/or underlayers 4 (e.g. Cr or a Cralloy underlayer), one or more magnetic layers 5 (e.g. a Co alloylayer), and one or more protective overcoats 6 (e.g. one or more carbonprotective overcoat layers). The texture typically comprises scratchesin the circumferential direction (or generally in the circumferentialdirection) formed using polycrystalline or mionocrystalline diamondparticles having a size of about 0.05 to 0.5 μm in diameter. The texturetypically accomplishes several functions, including facilitating theflying of a read-write held above the magnetic disk. However, moreimportantly, the texture causes the magnetic characteristics of the diskto become anisotropic. For example, the coercivity Hc of the magneticlayer is higher in the circumferential direction than the radialdirection. Also, the magnetic remanence Mr is higher in thecircumferential direction than the radial direction.

It has been a trend in magnetic disk manufacturing that disks have beengetting smoother and smoother. This is in part because it is necessaryto permit the read-write head to fly closer and closer to the magneticlayer. Current disks have a Ra of about 3 to 5 Å as measured by anatomic force microscope (“AFM”). (Ra is a well-known measure ofroughness.) Unfortunately, as disks become smoother, and as the Ra dropsbelow 5 Å, the magnetic anisotropy of the disk drops. Further, othermagnetic characteristics of the disk are degraded, e.g. the signal tonoise ratio (“SNR”) and the pulse width PW50.

In prior art diamond texturing, the diamond particles of texturingslurries have highly variable sizes. Because of this, the resultingtexture scratches have highly variable sizes (i.e. variable widths anddepths). This, in turn, can cause a) magnetic defects (e.g. because theeffective flying height between the read-write head and magnetic layerincreases at locations where the texture scratches are too deep); and b)reliability problems caused by the fact that deep scratches can make itdifficult to passivate the disk. Further, the presence of oversizedparticles in the texturing slurry results in poor glide characteristicsbecause excessively high ridges can be formed adjacent excessively deepscratches.

SUMMARY

During a method in accordance with the present invention, “nano-sized”particles are used to texture a substrate. The particles are typicallydiamond, and typically have a diameter less than about 20 nm. In oneembodiment, the particle diameters are less than about 10 nm. Theparticle diameters are typically greater than about 1 nm, and in oneembodiment greater than about 2 nm. In one embodiment, they are between3 and 8 nm. (The word “diameter” as used herein does not require thatthe particles are spherical.) The particles are typicallymonocrystalline. In one embodiment, they are used to form scratch lineshaving a density greater than 50 lines per micron.

After texturing, one or more layers are deposited on the substrate,including a magnetic alloy layer (e.g. a Co or Fe based alloy layer) tothereby form a magnetic data storage medium.

We have discovered that by using “nano-sized particles” (e.g. nano-sizeddiamond particles) for texturing a magnetic disk we can form extremelysmooth textures without sacrificing such magnetic characteristics asanisotropy, SNR and FW50.

In one embodiment, the slurry has generally uniform particle sizes (e.g.between 3 and 8 nm). Because of this, the texture lines formed by theslurry will have uniform widths and depths. This is important because itis desirable to avoid gouges (e.g. to avoid magnetic defect andpassivation problems discussed above) and to avoid forming high ridges(e.g. to thereby avoid glide height problems). Further, the nano-sizeddiamond particles also result in a reduction of large low frequencytexture lines. Also, the scratch size and density of scratch lines helpone to control the magnetic layer grain size. (It is desirable to form amagnetic layer comprising small uniform grain sizes.)

In one embodiment, the nano-sized particles are free abrasive particles,e.g. within a liquid (for example an aqueous slurry). In anotherembodiment, the nano-sized particles are fixed abrasive particles, e.g.bound to an abrasive cloth or other structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates in cross section a prior art magneticdisk.

FIGS. 2A and 2B schematically illustrate in plan view and cross-sectionapparatus for texturing a magnetic disk substrate.

FIG. 3 illustrates the relationship between MrT OR (the MrT orientationratio) vs. roughness Ra for disks comprising substrates textured withconventional diamond particles and nano-sized diamond particles. FIG. 3also illustrates the relationship between SNR and roughness Ra for diskscomprising substrates textured with conventional diamond particles andnano-sized diamond particles.

FIGS. 4A, 4B and 4C are SEM photographs of prior art diamond texturingparticles.

FIGS. 5A and 5B are SEM photographs of nano-sized diamond particles usedduring a method in accordance with the present invention.

FIG. 6A is an AFM photograph of a first substrate textured using priorart diamond texturing particles.

FIG. 6B is a cross section AFM scan of the substrate of FIG. 6A.

FIG. 6A′ is an AFM photograph of a second substrate textured using priorart diamond texturing particles.

FIG. 6B′ is a cross section AFM scan of the substrate of FIG. 6A′.

FIG. 7A is an AFM photograph of a substrate textured using nano-sizeddiamond texturing particles.

FIG. 7B is a cross section AFM scan of the substrate of FIG. 6A.

FIG. 8 illustrates a magnetic disk drive comprising a disk formed usinga method in accordance with our invention.

DETAILED DESCRIPTION

FIGS. 2A and 2B illustrate apparatus 11 for texturing a substrate 12.(Apparatus 11 is not novel in and of itself.) In FIGS. 2A and 2B, amotor (not shown) rotates substrate 12 while a sheet 13 of material(typically nylon) is pushed against substrate 12. (Sheet 13 moves off ofa supply reel 14, around a roller 15, and onto a take-up reel 16 (seearrow B).) Roller 15 urges sheet 13 against substrate 12, e.g. with aforce between 1 and 10 pounds. In one embodiment, the force is between2.5 and 5 pounds. A slurry comprising nano-sized diamond particles(described below) is introduced between sheet 13 and substrate 12.Simultaneously, sheet 13 and roller 15 reciprocate, moving back andforth in a direction A. (Alternatively, substrate 12 can reciprocateinstead of sheet 13 and roller 15.) The diamond particles form texturescratches that are non-random, and are in a generally circumferentialdirection in substrate 12. Because of the motion of sheet 13 and roller15 in the direction of arrow A, the resulting texture exhibits across-hatch. (The texture lines therefore intersect, e.g. at an anglebetween 0 and 20 degrees, and in one embodiment a range of 2 to 6degrees, although these values are merely exemplary.) In one embodiment,apparatus 11 is device model no. 1800, available from EDC Corporationlocated in California. The slurry flow rate is typically between 0.1 and1 ml/second. The above-described apparatus and parameters are merelyexemplary. Other types of texturing apparatus and parameters can also beused.

In one embodiment, the slurry comprises a commercial coolant orlubricant and between 0.4 to 1 gram/liter of diamond particles, e.g.about 0.4 grams per liter of diamond particles. As mentioned above, thediamond particles can have a diameter from 2 to 8 nm.

As mentioned above, the size and spacing of the texture marks depend atleast in part on the size of the diamond particles. Using particleshaving sizes greater than or equal to 2 nm helps ensure a certainminimum grain size in the subsequently formed magnetic layer. (if thegrains are too small, their magnetization state may be thermallyunstable. However, in other embodiments, diamond particles less than 2nm in size can be used.)

Description of Types of Nano-Diamond Particles Used With the Invention

In one embodiment, the particles can be formed using a method asdescribed by Vereschagin et al. in U.S. Pat. No. 5,861,349, incorporatedherein by reference. Such particles are available from UltradiamondTechnologies, Inc. of Somerville, Mass. (e.g. product no. UD90).Alternatively, the particles can be of the type available fromPlasmaChem of Mainz, Germany. They are typically formed at a hightemperature and pressure with an explosion. In one embodiment, thediamond particles comprise about 90% or more of diamond, with some ashand/or oxidatable carbon making up the remainder.

Manufacture of a Magnetic Disk Comprising a Substrate Textured Using thePresent Invention

After texturing, one of more underlayers, magnetic layers and protectiveovercoats are applied to the disk. In one embodiment, the underlayersand magnetic layers can be as described in U.S. patent application Ser.No. 10/075,123, filed by Bertero et al. and incorporated herein byreference or U.S. Pat. No. 6,150,015, issued to Bertero at al. and alsoincorporated herein by reference. The overcoat layers can be asdescribed in U.S. patent application Ser. No. 09/604,490, filed byLairson et al. or German patent document DE 101 30 942 A1, each beingincorporated herein by reference. However, the specific processesdescribed by Bertero and Lairson are merely exemplary, and otherprocesses could also be used. For example, other disk manufacturingprocesses in which one or more ferromagnetic layers (e.g. Co or Fe basedmagnetic layers) are deposited by a vacuum deposition process (e.g.sputtering) can be used. Also, other types of layers can be deposited onthe substrate during the disk manufacturing process.

Properties of a Magnetic Disk Using a Substrate Textured WithNano-Diamond Particles

A magnetic disk manufactured using nano-sized diamond particles exhibitsseveral surprising and unique characteristics. As mentioned above, priorto the present invention, substrates were textured using diamondparticles having a diameter between 0.05 and 0.5 μm. FIG. 3 illustratesthe relationship between the MrT orientation ratio (MrT OR) for diskscomprising substrates manufactured using such particles. (MrT equalsmagnetic remanence times thickness of the magnetic layer.) Curve 22shows the MrT OR for disks manufactured using diamond particles inaccordance with the prior art, whereas curve 24 shows the MrT OR fordisks manufactured using nano-sized diamond particles (in this casehaving diameters between 3 and 8 nm with an average diameter of 5 nm).As can be seen, when using the prior art texturing particles (curve 22),MrT OR dropped precipitously when the Ra dropped below about 2.2 Å.Thus, one skilled in the art, using these particles, would be led tobelieve that one could not use a Ra less than 2.2 Å without sacrificingMrT OR. However, as demonstrated in curve 24, we have discovered thatwhen using nano-sized diamond particles, one can maintain MrT OR high(e.g. about 1.6) even for Ra's of 1.4 to 1.5 Å. This is an important andsurprising discovery.

FIG. 3 also shows that for a magnetic disk textured with the prior artdiamond particles, when the Ra dropped below 2.2 Å, the SNR droppedprecipitously. (See curve 26.) Surprisingly, when one textures thesubstrates with nano-sized diamond particles, even at a Ra of about 1.4to 1.5 Å, the SNR remained high—about 0.45 dB. (See curve 28.) Again,this result was completely unexpected.

FIGS. 4A, 4B and 4C are SEM photographs of prior art diamond texturingparticles at 60,000 ×, 300,000 × and 500,000 × magnification,respectively, whereas FIGS. 5A and 5B are SEM photographs of nano-sizeddiamond texturing particles used during a texturing method in accordancewith the invention at magnifications of 60,000 and 500,000 ×,respectively. As can be seen, the prior art particles are larger thanthe nano-sized particles. Further, there is considerable size variationin the prior art particles, and the prior art particles have numerousjagged and irregular facets. This leads to highly variable andundesirable scratch characteristics.

As mentioned above, FIGS. 5A and 5B are SEM photographs of nano-sizeddiamond particles used to texture magnetic disks in accordance with thepresent invention. As can be seen, the particles are small, have roughlythe same size. As discussed above, the small size and uniformity ofparticles permits one to form small, uniform scratch marks.

FIGS. 6A and 6A′ are AFM photographs of first and second substratestextured using prior art diamond particles. FIGS. 6B and 6B′ are AFMcross section scans of the substrates of FIGS. 6A and 6A′, respectively.FIGS. 7A and 7B are an AFM photograph and cross section scan,respectively, of a substrate textured using nano-sized diamondparticles. Table 1 below lists various parameters relating to thesurface finish of the substrates of FIGS. 6. 6′ and 7. TABLE I(Distances in angstroms) Distance between Distance between mean surfacemean surface Substrate AFM Ra and peaks and valleys 3.9 16.8 32.0 FIG.6A′, 6B′ 2.8 14.9 23.9 1.6 8.2 13.2

As can be seen, the scratch lines of the substrate of FIGS. 7A and 7Bare shallower and more regular compared to the scratch lines of FIGS.6A, 6B, 6A′ and 6B′. As mentioned above, this has the followingadvantages: a) the effective flying height of a read-write head withrespect to the magnetic layer is reduced; b) there are fewer or no deepgouges for causing magnetic defects; c) the substrate of FIGS. 7A and 7Bis easier to passivate; d) there are fewer or no ridges for a read-writehead to collide with; and e) the texture grooves of FIGS. 7A and 7Bfacilitate forming smaller magnetic grains.

In one embodiment, the texture scratch lines formed using a method inaccordance with the invention have a density greater than or equal to 50per micron and less than or equal to 150 per micron. In one embodiment,the scratch density is between about 50 and 120 per micron, e.g. about80 per micron. (Typically, the minimum scratch density is inverselyproportional to the particle diameter, whereas the maximum scratchdensity is inversely proportional to {fraction (1/10)} of the particlediameter.)

INDUSTRIAL APPLICATION

Magnetic disks are incorporated into disk drives, e.g. disk drive 30 ofFIG. 8. Referring to FIG. 8, disk 32 is mounted on a spindle 33, whichis rotated by a motor 34. A pair of read-write heads 35 a, 35 b are heldon suspensions 36 a, 36 b, which in turn are mounted on an actuator 38for moving heads 35 a, 35 b over the various tracks of disk 32. Duringuse, heads 35 a, 35 b “fly” above disk 32, and are used to read datafrom or write data to disk 32. (It will be appreciated that both sidesof disk 32 are textured, and the various layers used to manufacture amagnetic disk are formed on both sides of disk 32, although in otherembodiments, one can texture and deposit these layers on only one sideof disk 32.)

While the invention has been described with respect to specificembodiments, those skilled in the art will appreciate that changes canbe made in form and detail without departing from the spirit and scopeof the invention. For example, different types of substrates can betextured using the above-described techniques. Thus, the substrate canbe an aluminum alloy disk coated with a nickel phosphorus alloy.However, materials other than NiP and aluminum alloys can be used (e.g.glass or glass ceramic substrates), and substrate shapes other thandisks can be used (e.g. for media that is not disk-shaped). Differenttypes of texturing apparatuses, with different parameters, can also beused. Different sized particles (e.g. having a size from 0.5 to 20 nm)can be used.

While the method of the present invention can be used to manufacturedisks used in longitudinal magnetic recording, the method can also beused to form disks used in vertical recording. (As mentioned above,controlling the scratch sizes can be used to control or influence themagnetic layer grain size. This is useful in both longitudinal andvertical recording.) The method of the present invention can also beused to form isotropic media. While the texturing particles aretypically diamond, other hard materials can also be used. Accordingly,all such changes come within the invention.

1-28. (canceled)
 29. A method comprising texturing a substrate withparticles having a size less than or equal to about 20 nm to texturesaid substrate, said texturing resulting in a Ra of said substrate lessthan 2.2 Å.
 30. Method of claim 29 wherein said particles comprisediamond.
 31. Method of claim 29 further comprising forming a magneticlayer above said substrate.
 32. Method of claim 31 further comprisingforming one or more intermediate layers between said substrate and saidmagnetic layer.
 33. Method of claim 32 wherein said abrasive particleshave a diameter greater than or equal to 2 nm.
 34. Method of claim 32wherein said texture causes said magnetic layer to exhibit anisotropicmagnetic characteristics.
 35. Method of claim 32 further comprisingmounting said substrate with said magnetic layer formed thereon in adisk drive.
 36. A method comprising texturing a magnetic data storagemedium substrate with particles being sufficiently small to form atexture Ra less than 2.2 Å while inducing magnetic anisotropy in amagnetic layer deposited over said substrate, said texturing resultingin a Ra of said substrate less than 2.2 Å.
 37. Method of claim 36further comprising depositing said magnetic layer over said substrate.38. Method of claim 37 further comprising one or more intermediatelayers between said substrate and said magnetic layer.
 39. Method ofclaim 38 further comprising mounting said substrate with said magneticlayer formed thereon in a disk drive.
 40. Structure comprising: asubstrate textured with abrasive particles having a size less than orequal to about 20 nm and having a Ra less than 2.2 angstroms. 41.Structure of claim 40 further comprising a magnetic recording layerformed above said substrate, said magnetic recording layer exhibitinganisotropy.
 42. Structure of claim 41 further comprising one or moreintermediate layers formed between said substrate and said magneticlayer.
 43. A magnetic disk drive comprising the structure of claim 41.44. A magnetic data storage medium substrate textured with abrasiveparticles sufficiently small to leave a Ra less than 2.2 angstroms whilepermitting a magnetic layer deposited over said substrate to exhibitmagnetic anisotropy.
 45. Structure of claim 44 further comprising amagnetic layer formed over said substrate, said layer exhibitingmagnetic anisotropy.
 46. Structure of claim 45 further comprising one ormore intermediate layers formed between said substrate and said magneticlayer.